Method and apparatus for recording to and reading from a diffractive optics memory using symmetrical angular encoding

ABSTRACT

The present invention relates to an apparatus and method for reading information from and recording information to a diffractive optics memory ( 8 ) using symmetrical angular encoding. A coherent light source (LASER  10 ) is split to form an object beam (OBJ.B 4 ) and a corresponding reference beam (REF.B 1 ). An optical axis is defined by the object beam being aligned perpendicular to a plane of the diffractive optics memory. A steering mirror (R.M  40 ) is configured to direct the reference beam received from the coherent light source to the memory. A first plurality of mirrors ( 37   a ) arranged around one side of the optical axis receives the reference beam from the steering mirror and directs the reference beam at a first angle towards the memory. A second plurality of mirrors ( 37   b ) arranged around the symmetrical side of the optical axis receives the reference beam from the steering mirror and directs the reference beam at a second angle towards the memory. The first angle is identical to the second angle but formed on the symmetrical side of the optical axis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the applications entitled:

-   -   PHOTONICS DATA STORAGE SYSTEM USING A POLYPEPTIDE MATERIAL AND        METHOD FOR MAKING SAME (PCT/FR01/02386) filed on Jul. 20, 2001.

FIELD OF INVENTION

The present invention generally relates to a large volume diffractiveoptics memory. In particular, the present invention relates to anapparatus and method for recording information to and readinginformation from a diffractive optics memory.

BACKGROUND OF THE INVENTION

The large storage capacities and relative low costs of CD-ROMS and DVDshave created an even greater demand for still larger and cheaper opticalstorage media. Holographic memories have been proposed to supersede theoptical disc as a high-capacity digital storage medium. The high densityand speed of the holographic memory comes from three-dimensionalrecording and from the simultaneous readout of an entire packet of dataat one time. The principal advantages of holographic memory are a higherinformation density (10¹¹ bits or more per square centimeter), a shortrandom access time (˜100 microseconds and less), and a high informationtransmission rate (10⁹ bit/sec).

In holographic recording, a light beam from a coherent monochromatic ormultispectral source (e.g., a laser) is split into a reference beam andan object beam. The object beam is passed through a spatial lightmodulator (SLM) and then into a storage medium. The SLM forms a matrixof shutters (in the binary case) or, more generally, a matrix ofphotocells modulating the light intensity that represents a packet ofdata. The object beam passes through the SLM which acts to modulate theobject beam with the binary information being displayed on the SLM. Themodulated object beam is then directed to one point on the storagemedium by a beam processor where it intersects with the reference beamto create a hologram representing the packet of data.

An optical system consisting of lenses and mirrors is used to preciselydirect the optical beam encoded with the packet of data to theparticular spatially addressed area of the storage medium. Optimum useof the capacity of a thick storage medium is realized by spatial andangular multiplexing. In spatial multiplexing, a set of packets isstored in the storage medium shaped into a plane as an array ofspatially separated and regularly arranged sub-holograms by varying thebeam target in the x-axis and y-axis of the plane. Each sub-hologram isformed at a point in the storage medium with the rectangular coordinatesrepresenting the respective packet address as recorded in the storagemedium. In angular multiplexing, recording is carried out by keeping thex- and y-coordinates the same while changing the irradiation angle ofthe reference beam in the storage medium. By repeatedly incrementing theirradiation angle, a plurality of packets of information is recorded asa set of sub-holograms at the same x- and y-spatial location.

Previous techniques for recording information in a highly multiplexedvolume holographic memory and for reading the information out of theholographic memory are limited in memory capacity.

It is therefore an object of the present invention to provide anapparatus for recording information to a memory capable of an extendedstorage capacity.

It is also an object of the present invention to provide an apparatusfor reading a memory capable of an extended storage capacity.

It is a further object of the present invention to double the capacityof the memory storage.

Further objects and advantages of the present invention will becomeapparent from a consideration of the drawings and ensuing description.

SUMMARY OF THE INVENTION

In order to achieve the above-mentioned objectives, the presentinvention comprises a diffractive storage system for recordinginformation on a diffractive optics memory. A coherent light source issplit to form an object beam and a corresponding reference beam. Anoptical axis is defined by the object beam being aligned perpendicularto a plane of the diffractive optics memory. A steering mirror isconfigured to direct the reference beam received from the coherent lightsource to the memory. A first plurality of mirrors arranged around oneside of the optical axis receives the reference beam from the steeringmirror and directs the reference beam at a first angle of a plurality offirst angles towards the memory. A second plurality of mirrors arrangedaround the symmetrical side of the optical axis receives the referencebeam from the steering mirror and directs the reference beam at a secondangle of a plurality of second angles towards the memory. The firstangle is identical to the second angle but formed on the symmetricalside of the optical axis.

In a further aspect of the present invention, the steering mirror is arotating mirror.

In yet another aspect of the present invention, the steering mirror is aMicro Opto Electro Mechanical System.

A further aspect of the present invention comprises the memory having aplurality of points storing information therein. The object beam and thereference beam interfere at the first angle to form a first sub-hologramat one of the points of the memory and the reference beam interfereswith the object beam at the second angle to form a second sub-hologramat the point.

In another aspect of the present invention, the steering mirror islocated on the optical axis which directs the reference beam to one ofthe mirrors.

In still another aspect of the present invention, the memory is made ofa polypeptide material.

In yet another aspect of the present invention, the present inventioncomprises a diffractive storage system for reading information from adiffractive memory. A coherent light source forms a reference beam. Anoptical axis is defined by the reference beam being alignedperpendicular to a plane of the memory. A steering mirror is configuredto direct the reference beam received from the coherent light source tothe memory. A first plurality of mirrors arranged around one side of theoptical axis receives the reference beam from the steering mirror anddirects the reference beam at a first angle of a plurality of firstangles towards one of the points of the memory. A second plurality ofmirrors arranged around the symmetrical side of the optical axisreceives the reference beam from the steering mirror and directs thereference beam at a second angle of a plurality of second angles towardsthe one of the points of the memory. The first angle is the same valueas the second angle but formed on the symmetrical side of the opticalaxis.

In yet another aspect of the present invention, an array of lightsensitive elements is configured to detect a reconstruction of a firstpacket of information at the point of the memory illuminated with thereference beam and to detect a reconstruction of a second packet ofinformation at the point of the memory illuminated with the referencebeam

In still another aspect of the present invention, the optical axis isdefined perpendicular to a plane of the memory by the object beam.

In a further aspect of the present invention, the first angle isidentical in value to the second angle but formed on the symmetricalside of the optical axis.

In yet another aspect of the present invention, a steering mirrordirects the reference beam to any of the first and second plurality ofmirrors.

In still another aspect of the present invention, the steering mirror isa Micro Opto Electro Mechanical System.

In a further aspect of the present invention, the steering mirror islocated on the optical axis directing the reference beam to one of theplurality of mirrors.

In still another aspect, the present invention comprises the memorywherein the object beam and the reference beam interferes at the firstangle to form a sub-hologram at a point of the storage memory and thereference beam interferes with the object beam at the second angle toform a second sub-hologram at the point.

In another aspect of the present invention, the memory is made of apolypeptide material.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present invention,reference is now made to the appended drawings. These drawings shouldnot be construed as limiting the present invention, but are intended tobe exemplary only.

FIG. 1 shows an apparatus for recording information on a diffractiveoptics memory according to the present invention.

FIG. 2 shows the process of diffractive recording by interference of anobject beam and a reference beam.

FIG. 3 is a schematic representation of a matrix of points formed in adiffractive optics memory

FIG. 4 shows an apparatus for reading information from a diffractiveoptics memory according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Recording Apparatus

FIG. 1 shows an apparatus 100 for recording information on a diffractiveoptics memory 8 according to the present invention. A laser 10 emits acoherent light beam which is divided by a beam splitter 20 into areference beam 1 and an object beam 4. The object beam 4 is directed bymirror 25 to the diffractive optics memory 8. The diffractive opticsmemory 8 comprises a recording plate coated with a layer of polypeptide.After the object beam 4 reflects off of the mirror 25, the object beam 4is processed by a spatial filter (S.F.) 27 and a collimating lens (C.L.)28 so that it transmits through display 29 where it is modulated with apacket of information 6 (see FIG. 2) an focused by focussing lens 31onto the recording plate 8. The mirrors 35, 40 are located in differentplanes from the object beam 4 so as not to obstruct the object beam 4.The spatial filter 27 removes unwanted noise from the laser beam througha simple diffractive phenomena. Generally, the spatial filter 27 iscomposed of a short focal lens with a pinhole located in its focussingplane. The laser light out of this hole is cleaned (smoothed) from allthe beam defects so that the spatial light modulator (SLM) 29 will beilluminated with a uniform laser light. The collimating lens 28(symbolized by a double arrow) collimates the laser beam coming from thespatial filter 27 to transform a divergent shaped beam to a collimatedbeam so that it reaches a uniform intensity on the display 29, that is,in a way that the light reaching any cell of the display 29 will beequal.

The display 29 may be any display for displaying a data packet 6 in twodimensions such as a spatial light modulator (SLM) or a liquid crystallight valve (LCLV). The display 29 comprises, for example, a liquidcrystal display screen on which data is encoded in a two-dimensionalpattern of transparent and opaque pixels. The data is input to thedisplay 29 via a computer (not shown) or by other digital data or analogorigins. The plurality of bits represented on the display screen of thedisplay 29, a two-dimensional pattern of transparent and opaque pixels,is known as a data packet 6 (see FIG. 2). The data packed 6 displayed isderived from any source such as a computer program, the Internet, or anyother data source. In an Internet storage application, the packetsdisplayed may be formatted similarly to the packets of the Internet. Theobject beam 4 is modulated by the information to be recorded by means oftransmission through the display 29.

At the same time, the reference beam 1 undergoes various reflections offthe set of mirrors 30, 35, 40 at least one of which can rotate so thatthe reference beam 1 arrives at a plurality of micro-mirrors 37 a, 37 bwhich are distributed along a circular arc and the orientation of whichwill modify the angle of incidence of the reference beam 1 with respectto the object beam 4, again in the region of the diffractive opticsmemory 8. Thus, by this process, angular multiplexing is implemented.The recording apparatus 100 implements symmetrical addressing by angularmultiplexing on both sides of the optical axis of the object beam 4. Theoptical axis is formed by that segment of the object light beam 4positioned between mirror 25 and the diffractive optics memory 8 so thatit is perpendicular to a plane of the diffractive optics memory 8. Thefirst plurality of mirrors 37 a arranged around one side of the opticalaxis receives the reference beam 1 from the steering mirror 40 and oneof the first plurality of mirrors 37 a then directs the reference beamat a first angle of a plurality of first angles towards the memory 8.The second plurality of mirrors 37 b arranged around the symmetricalside of the optical axis receives the reference beam 1 from the steeringmirror 40 and one of these mirrors then directs the reference beam 1 ata second angle of a plurality of second angles towards the memory 8. Thefirst angle is identical to the second angle but formed on thesymmetrical side of the optical axis.

The diffractive optics memory 8 comprises a recording plate havingcoated thereon a polypeptide photosensitive material. As illustrated inFIG. 1, reference light beam 11 and reference light beam 12 are formedto intersect the optical axis at a point of the diffractive opticsmemory 8 at an identical angle but on opposite sides of the opticalaxis. The optical axis is formed by the object beam 4 as shown in FIG. 1and described above. There is formed a diffracted optical image 8 a (seeFIG. 2), or more precisely a structure resulting from the interferenceof the object beam 4 with the reference beam 1, which is stored in thestorage material 8. Spatial multiplexing is carried out by mechanicallyshifting the material 8 so that a data packet is recorded at a differentpoint of the material 8.

FIG. 2 shows a schematic of the important signals involved in recordinga diffraction pattern, that can be named alternately a hologram, in athe diffractive optics memory 8 using angular and spatial multiplexing.Various diffractive recording processes have been developed in the artand further details can be found in the book Holographic Data Storage,Springer (2000) edited by H. J. Coufal, D. Psaltis, and G. T. Sincerbox.In forming a hologram, the reference beam 1 intersects with the objectbeam 4 to form a sub-hologram 8 a (referred to alternately as a point)extending through the volume of the memory 8. There is a separatesub-hologram or point 8 a extending through the volume for each angleand spatial location of the reference beam 1. The object beam 4 ismodulated with a packet of information 6. The packet 6 containsinformation in the form of a plurality of bits. The source of theinformation for the packet 6 can be a computer, the Internet, or anyother information-producing source. The hologram impinges on the surface8 a of the storage medium 8 and extends through the volume of thestorage medium 8. The information for the packet 6 is modulated onto thestorage medium 8 by spatial multiplexing and angle multiplexing. Anglemultiplexing is achieved by varying the angle a of the reference beam 1with respect to the surface plane of the storage medium 8. A separatepacket 6 of information is recorded in the storage medium 8 as asub-hologram for each chosen angle a and spatial location. Spatialmultiplexing is achieved by shifting the reference beam 1 and the objectbeam 4 with respect to the surface of the storage medium 8 (achieved bytranslating the recording plate) so that the point 8 a shifts to anotherspatial location, for example point 8 a′, on the surface of the storagemedium 8.

The storage medium 8 is typically a three-dimensional body made up of amaterial sensitive to a spatial distribution of light energy produced byinterference of the object light beam 4 and the reference light beam 1.A hologram may be recorded in a medium as a variation of absorption orphase or both. The storage material must respond to incident lightpatterns causing a change in its optical properties. In a volumehologram, a large number of packets of data can be superimposed asdiffraction patterns, so that every packet of data can be reconstructedwithout distortion. A volume (thick) hologram may be regarded as asuperposition of three dimensional gratings recorded in the depth of theemulsion each satisfying the Bragg law (i.e., a volume phase grating).The grating planes in a volume hologram produce change in refractionand/or absorption.

Several materials have been considered as storage material for opticalstorage systems because of inherent advantages. These advantages includea self-developing capability, dry processing, good stability, thickemulsion, high sensitivity, and nonvolatile storage. Some materials thathave been considered for volume holograms are photofractive crystals,photopolymer materials, and polypeptide material.

The diffractive optics memory 8 may be made of photopolymer materials,polypeptide material, and other such materials for optical recording.With a photopolymer the density storage will be much more limited thanby using a polypeptide with a shorter life duration and a lower SNR anda lower tolerancing. Thus, preferably, the diffractive optics memory 8is made of a polypeptide material. An embodiment of a polypeptidematerial suitable for the storage medium 8 is disclosed in theapplication PHOTONICS DATA STORAGE SYSTEM USING A POLYPEPTIDE MATERIALAND METHOD FOR MAKING SAME (PCT/FR01/02386) filed on Jul. 20, 2001 andincorporated herein.

FIG. 3 shows in greater detail the diffractive optics memory (i.e.,storage medium) 8 arranged in the form of a flat sheet, herein referredto as a matrix. In this example, the matrix is 1 cm². Each of aplurality of points on the matrix is defined by its rectilinearcoordinates (x, y). An image-forming system (not shown) reduces theobject beam 4 to the sub-hologram 8 a having a minimum size at one ofthe x, y points of the matrix. A point in physical space defined by itsrectilinear coordinates contains a plurality of packets 8 b.

In this embodiment, a 1 mm² image 8 a is obtained by focusing the objectbeam 4 onto the storage medium 8 centered at its coordinate. Due to thisinterference between the two beams 1,4, a diffractive pattern 8 a 1 mm²in size is recorded in the storage material 8 centered at thecoordinates of the matrix. Spatial multiplexing is carried out bysequentially changing the rectilinear coordinates. The object beam 4focuses on the storage material 8 so that a separate pattern 8 a isrecorded at a unique position in the plane defined by its coordinates(x, y). This spatial multiplexing results in a 10 by 10 matrix ofdiffractive images 8 a. Angle multiplexing is carried out bysequentially changing the angle of the reference beam 1 by means of themirrors 37 a, 37 b as described above. Angle multiplexing is used tocreate 15-20 packets of information 8 b corresponding to 15 discretevariations of the angle of incidence of the reference beam. Experimentalresults show that 25 multiplexing angles are possible and this can bedoubled, by the symmetric set-up of the present invention to 50 angles.A data packet is reconstructed by shinning the reference beam 1 at thesame angle and spatial location in which the data packed was recorded.The diffractive portion of the reference beam 1 diffracted by thestorage material 8 forms the reconstruction, which is typically detectedby a detector array. The storage material 8 may be mechanically shiftedin order to store data packets at different points by its coordinates(x, y).

Reading Apparatus

FIG. 4 shows an apparatus 200 for reading information from thediffractive optics memory 8 according to the present invention. A laser110 emits a coherent light reference beam 1 which is directed by mirror130 to a mirror 135. Mirror 135 then directs the coherent light beam toMEOMS (Micro Opto Electro Mechanical System) 140. The MEOMS 140 isconfigured to direct the reference beam 1 to any one of a plurality ofmicro-mirrors 137 a, 137 b which are distributed along a circular arcand the orientation of which will modify the angle of incidence of thereference beam 1. An optical axis is defined by the reference beam 1being aligned perpendicular to a plane of the memory 8. The firstplurality of mirrors 137 a arranged around one side of the optical axisreceives the reference beam 1 from the steering mirror 140 and one ofthese mirrors then directs the reference beam 1 at a first angle of aplurality of first angles towards one of the points of the memory 8. Thesecond plurality of mirrors 137 b arranged around the symmetrical sideof the optical axis receives the reference beam 1 from the steeringmirror 140 and one of these mirrors then directs the reference beam 1 ata second angle of a plurality of second angles towards the one of thepoints of the memory 8. The first angle is the same value as the secondangle but formed on the symmetrical side of the optical axis.

A data packet is reconstructed by positioning the reference beam 1 atthe same angle and spatial location in which the data packet wasrecorded. The portion of the reference beam 1 diffracted by thediffractive optics memory 8 forms the reconstruction, which is focusedby imagining lens 150 to a detector array of CCD camera 160.

The principal of symmetrical angle reading is illustrated with thereference beams 11, 12. The MEOMS 140 is steered by a computer program(not shown) to form reference beam 11. The MEOMS 140 is then steered bythe computer program to form the reference beam 12 at a symmetricalangle. The reference light beam 11 and reference light beam 12 intersectthe optical axis at a point of the diffractive optics memory 8 at anidentical angle value but on opposite sides of the optical axis. Thustwo separate packets of information are sequentially reconstructed fromthe same point with symmetrical angles.

The present invention is not to be limited in scope by the specificembodiments described herein. Indeed, this application is intended tocover and modifications of the present invention, in addition to thosedescribed herein, and the present invention is not confined to thedetails which have been set forth. Thus, the scope of the inventionshould be determined by the appended claims and their legal equivalents,rather than by the examples given.

1. A diffractive data storage system for recording data from a data source on diffractive optics memory, comprising: a coherent light source split to form an object beam and a corresponding reference beam, the object beam being modulated by the data by means of transmission through a display encoding the data inputted by the data source in a two-dimensional pattern of transparent and opaque pixels and focused on said memory following an optical axis perpendicular to a plane of said memory a steering mirror configured to direct said reference beam received from said coherent light source; a first plurality of mirrors arranged around one side of said optical axis receiving said reference beam from said steering mirror, each of said first plurality of mirrors directing said reference beam at a corresponding first angle of a plurality of first angles towards said memory; and a second plurality of mirrors arranged around the symmetrical side of said optical axis receiving said reference beam from said steering mirror, each of said second plurality of mirrors directing said reference beam at a second angle of a plurality of second angles towards said memory, said first angle being identical in value to said second angle but formed on the symmetrical side of said optical axis; said memory comprising a plurality of points storing data therein, said object beam and said reference beam interfering at said first angle to form a first sub-hologram at one of said points of said memory and said reference beam interfering with said object beam at said second angle to form a second sub-hologram at said point, and said memory being mechanically shifted so that data are recorded at different points of said memory.
 2. The diffractive storage system of claim 1, wherein said memory comprises a polypeptide plate on which data is recorded.
 3. The diffractive storage system of claim 1, wherein said steering mirror is a rotating mirror.
 4. The diffractive storage system of claim 1, wherein said steering mirror is a Micro Opto Electro Mechanical System.
 5. The diffractive storage system of claim 1, wherein the display is a spatial light modulator.
 6. the diffractive storage system of claim 1, wherein the display is a liquid crystal light wave.
 7. The diffractive storage system of claim 1, wherein said memory is made of a polypeptide material.
 8. The diffractive storage system of claim 1 wherein the steering mirror is placed between said display and said memory
 9. A diffractive storage method for recording data from a data source on a diffractive optics memory, comprising the steps of: forming an object beam and a reference beam coherent with said object beam; modulating the object beam by the data by means of transmission through a display encoding the data inputted by the data source in a two-dimensional pattern of transparent and opaque pixels and focusing the object beam on said memory following an optical axis perpendicular to a plane of said memory directing said reference beam at a first angle of a first plurality of angles towards said memory by a corresponding one of a first plurality of mirrors arranged around one side of said optical axis; and directing said reference beam at a second angle of a second plurality of angles towards said memory by a corresponding one of a second plurality of mirrors arranged around the symmetrical side of said optical axis, said first angle being identical to said second angle but formed on the symmetrical side of said optical axis; said memory comprising a plurality of points storing data therein, said object beam and said reference beam interfering at said first angle to form a first sub-hologram at one of points of said memory and said reference beam interfering with said object beam at said second angle to form a second sub-hologram at said point, shifting said memory so that data are recorded at different points of said memory.
 10. The diffractive storage method of claim 9, further comprising a MEOMS which directs said reference beam to one of said plurality of mirror
 11. The diffractive storage method of claim 9, wherein said memory is made of a polypeptide material.
 12. The diffractive storage method of claim 9, wherein said object beam has modulated thereon a plurality of pixels.
 13. A diffractive data storage system for reading data from a diffractive optics memory having a plurality of points, comprising: a coherent light source forming a reference beam, an optical axis being defined by said reference beam being aligned perpendicular to a plane of said memory, a steering mirror configured to direct said reference beam received from said coherent light source to said memory; a first plurality of mirrors arranged around one side of said optical axis receiving said reference beam from said steering mirror, each of said first plurality of mirrors directing said reference beam at a corresponding first angle of a plurality of first angles towards one of said points of said memory, a second plurality of mirrors arranged around the symmetrical side of said optical axis receiving said reference beam from said steering mirror, each of said second plurality of mirrors directing said reference beam at a corresponding second angle of a plurality of second angles towards said one of said points of said memory, said first angle being the same value as said second angle but formed on the symmetrical side of said optical axis, and an array of light sensitive elements configured to detect a first reconstruction beam of a first packet of data at said point of said memory illuminated with said reference beam and to detect a second reconstruction beam of a second packet of data at said point of said memory illuminated with said reference beam.
 14. The diffractive storage system of claim 13, wherein said steering mirror is a Micro Opto Electro Mechanical System.
 15. The diffractive storage system of claim 13, wherein said steering mirror is located on said optical axis directing said reference beam to one of said plurality of mirrors.
 16. The diffractive storage system of claim 13, wherein said memory is made of a polypeptide material.
 17. A diffractive data storage method for reading data from a diffractive optics memory, comprising the steps of: directing a reference beam at a first angle of a first plurality of angles towards a first plurality of mirrors arranged around one side of an optical axis, said optical axis defined by said reference beam perpendicular to said memory, reconstructing a first packet of information at a point of said memory with said reference beam; detecting the first reconstructed packet with an array of light sensitive elements directing said reference beam at a second angle of a second plurality of angles towards a second plurality of mirrors, said first angle being identical in value and symmetrical about said optical axis to said second angle; reconstructing a second packet of information at said point of said memory with said reference beam, and detecting the second reconstructed packet with an array of light sensitive elements.
 18. The diffractive storage method of claim 17, wherein said memory is made of a polypeptide material. 